CALCAREOUS ALGAE OF A TROPICAL LAGOON DOCTORAL DISSERTATION IN PLANT PHYSIOLOGY

CALCAREOUS ALGAE OF A TROPICAL LAGOON
Primary Productivity, Calcification and Carbonate Production
JUMA WALAKU KANGWE
DOCTORAL DISSERTATION IN PLANT PHYSIOLOGY
DEPARTMENT OF BOTANY
STOCKHOLM UNIVERSITY
SWEDEN
2006
© 2006 Juma Kangwe
ISBN 91-7155-187-5
PrintCenter
Stockholm 2005
Front cover: A meadow of Halimeda opuntia exposed to air during lowest spring tides of the
day in Chwaka bay.
Back cover: Top: A mixed Halimeda meadow and Udotea species can be seen in the middle
(Photo by Katrin Österlund). Below: Rhodolith (left) and H. opuntia (right) meadows in
Chwaka bay.
2
To my parents;
The late father mzee Walaku Kangwe
My mummy Kuyeya Mpanjilwa
And my wife Mariana Kangwe
3
ABSTRACT
The green algae of the genus Halimeda Lamouroux (Chlorophyta, Bryopsidales) and the encrusting looselying red coralline algae (Rhodophyta, Corallinales) known as rhodoliths are abundant and widespread in all
oceans. They significantly contribute to primary productivity while alive and production of CaCO3 rich sediment
materials on death and decay. Carbonate rich sediments are important components in the formation of Coral
Reefs and as sources of inorganic carbon (influx) in tropical and subtropical marine environments. This study
was initiated to attempt to assess their ecological significance with regard to the above mentioned roles in a
tropical lagoon system, Chwaka bay (Indian Ocean), and to address some specific objectives on the genus
Halimeda (Chlorophyta, Bryopsidales) and the loose-lying coralline algae (rhodoliths).
Four Halimeda species were taxonomically identified in the area. The species identified are the most
common inhabitants of the world’s tropical and subtropical marine environments, and no new species were
encountered. Using Satellite remote sensing technique in combination with the percentage cover data obtained
from ground-truthing field work conducted in the area using quadrants, the spatial and seasonal changes of
Submerged Aquatic Macrophytes (SAV) were evaluated. SAV percentage cover through ground-truthing was;
24.4% seagrass, 16% mixed Halimeda spp., 5.3% other macroalgae species while 54.3% remained unvegetated.
No significant changes in SAV cover was observed for the period investigated, except in some smaller regions
where both loss and gains occurred. The structural complexity of SAV (shoot density, above-ground biomass
and canopy height) for most common seagrass communities from six meadows, dominated by Thalassia
hemprichii, Enhalus acoroides and Thalassodendron ciliatum, as well as mixed meadows, were estimated and
evaluated. Relative growth of Halimeda species was up to 1 segment tip-1 day-1. The number of segments
produced was highest in hot season. Differences between the numbers of segments produced were insignificant
between the two sites investigated. The C/N ratios obtained probably shows that Halimeda species experience
nitrogen limitation in the area and may be a factor among others responsible for the varying growth of species
obtained. However, this can be a normal ratio for calcified algae due to high CaCO3 content in their tissues.
Standing biomass of mixed Halimeda species averaged between 500-600 g dw m-2 over the bay, while the mean
cover in Halimeda meadows was about 1560 g dw m-2. Carbonate production in Halimeda beds varied between
17-57 g CaCO3 m-2 day-1 and for H. macroloba between 12-91 g CaCO3 m-2 day-1. This indicates a high annual
input of carbonate in the area. Decomposition of Halimeda using litter bag experiments at site I and II gave a
decomposition rate (k) of 0.0064 and k = 0.0091 day-1 ash-free dry weight (AFDW) respectively. Hence it would
take 76-103 days for 50% of the materials to decompose.
Adding inhibitors or varying the pH significantly reduced inorganic carbon uptake, and demonstrated that
the two photosynthesis and calcification were linked. Addition of TRIS strongly inhibited photosynthesis but not
calcification, suggesting the involvement of proton pumps in the localized low pH acid zones and high pH basic
zones. The high pH zones were maintained by the proton pumps maintaining high calcification, while TRIS was
competing for proton uptake from acid zones causing photosynthesis to drop. Rhodoliths were found to maintain
high productivity at a temperature of 34oC, and even at 37oC. It is therefore concluded that, rhodoliths are well
adapted to high temperatures and excess light, a behaviour which enables them to thrive even in intertidal areas.
Department of Botany
© Juma Walaku Kangwe
Stockholm University
ISBN 91-7155-187-5
Sweden
Doctoral Thesis
[email protected]
4
LIST OF PAPERS
This thesis is based on one published paper and three manuscripts. The papers will be
referred to by their roman numerals.
I
Martin Gullström, Bengt Lundén, Maria Bodin, Juma Kangwe, Marcus C. Öhman,
Matern S. P. Mtolera and Mats Björk (2005). Assessment of vegetation changes in
seagrass communities of tropical Chwaka Bay (Zanzibar) using satellite remote
sensing. (In press: Estuarine, Coastal and Shelf Science).
II
Kangwe, J.W. Mtolera, S.P.M. Kautsky, L and Björk, M. (2005). Growth and standing
biomass of Halimeda (Bryopsidales) species and their contribution to sediment
production in a tropical bay (In manuscript).
III
Kangwe, J.W. Mtolera, S.P.M. and Björk, M. (2005). Inorganic carbon uptake into
photosynthesis and calcification in two common Halimeda species. (In manuscript).
IV
Björk, M., Kangwe, J.W. and Mtolera, S.P.M. (2005). Temperature effects on
photosynthesis and calcification at varying light levels in rhodoliths from a tropical
lagoon (In manuscript).
Paper I is reproduced with the publisher’s permission. My contribution to the papers were:
(I) Performing vegetation assessments and ground-thruthing 2000 and 2001, as well as taking
part in writing; (II and III) Performing all experiments and surveys, taking major part in
planning and writing; (IV) Performing all experiments, taking part in planning and writing.
5
ABBREVIATIONS
ΔF/Fm´
Effective quantum yield
ΔF
Fm´- Ft
AF
Absorption factor
AZ
Acetazolamide – an inhibitor of external carbonic anhydrase
CA
Carbonic anhydrase
Ci
Inorganic carbon
DCMU
3-(3,4-dichlorophenyl)-1,1-dimethly-urea.
ETR
Electron transport rate at photosystem II and onwards to photosystem I
Fm
Maximal chlorophyll fluorescence in a dark adapted sample,
Fm’
As Fm, but in actinic light,
Fo
Minimal chlorophyll fluorescence in a dark adapted sample
Fo’
As Fo, but measured directly after an exposure to irradiance
Ft
Steady state chlorophyll fluorescence in actinic light
Fv
Variable fluorescence (F0-Fm)
Fv/Fm
Maximal quantum yield
IMS
Institute of Marine Sciences (in Zanzibar, Tanzania)
GPS
Global Positioning System
PAM
Pulse amplitude modulated
PAR
Photosynthetically active radiation
PSI
Photosystem I
PSII
Photosystem II
SAV
Submerged Aquatic Vegetation
TRIS
Tris (hydroxymethyl) aminomethane
UDSM
University of Dar es Salaam (in Tanzania)
6
TABLE OF CONTENTS
ABSTRACT ............................................................................................................................... 4
LIST OF PAPERS...................................................................................................................... 5
ABBREVIATIONS.................................................................................................................... 6
TABLE OF CONTENTS ........................................................................................................... 7
PREFACE .................................................................................................................................. 8
INTRODUCTION...................................................................................................................... 9
Study area ………………………………………………………………………………... 10
Algae .................................................................................................................................... 11
Calcifying algae.................................................................................................................... 12
The genus Halimeda............................................................................................................. 12
Reproduction in Halimeda ................................................................................................... 15
Rhodoliths………………………………………………………………………………….17
Photosynthesis and sources of inorganic carbon in aquatic environments .......................... 18
Algal calcification ................................................................................................................ 20
The link between algal photosynthesis and calcification ..................................................... 22
COMMENTS ON MATERIALS AND METHODS............................................................... 24
The use of Satellite Remote Sensing in assessing vegetation cover .................................... 24
Contributions from Halimeda species in the bay ................................................................. 25
Metabolic inhibitors of inorganic carbon uptake ................................................................. 26
Inhibitor – DCMU................................................................................................................ 27
Inhibitor – AZ....................................................................................................................... 28
Inhibitor – TRIS ................................................................................................................... 28
RESULTS AND DISCUSSION .............................................................................................. 29
CONCLUSIONS AND FUTURE PERSPECTIVES .............................................................. 35
ACKNOWLEDGEMENTS…………………………………………………………………. 36
REFERENCES......................................................................................................................... 38
7
PREFACE
Aragonite and calcite depositing calcareous algae are among the most abundant and
widely distributed seaweeds in the world’s oceans. They are the main contributors of primary
production while alive, and production of carbonate sediments in the marine environments
when they die (Bach, 1979; Hillis-Collinvaux, 1980; Drew, 1983; Multer, 1988; Payri, 1988).
In places without coral reefs, as in the Mediterranean, calcareous algae still play a major role
in the formation of biogenic deposits and build-ups of carbonate materials (Basso, 1998).
Little information however exists on species diversity in the Western Indian Ocean (WIO),
and most of studies on taxonomy, productivity and calcification (Borowitzka and Larkum,
1976b, c, d; Borowitzka,1977; Borowitzka, 1981; Wefer, 1980; Perry, 2005), growth and
sediment generation (Drew, 1983; Drew and Abel, 1985), bioherms (Davies and Marshall,
1985), ecology and distribution (Hillis-Collinvaux, 1980; Drew and Abel, 1988; Basso, 1998)
have been reported from other areas outside the WIO region.
Chwaka bay is in the east coast of Zanzibar in the WIO region (Fig. 1). Halimeda species
are exclusively flourishing over the area growing in substrates ranging from sandy, muddy to
rocky substrata. In soft bottom areas the soils are rich in dark decomposing Halimeda flakes
forming deep layers of mud which can reach over 3 meters (Muzuka, et al., 2001; Pers
obsers). However, their contribution to primary production and carbonate production in the
area has not been determined. The rhodoliths are mostly found in the western part of the
Chwaka bay towards Mapopwe creek, lying on a flat intertidal area (some in rocky pools)
mixed with the green algae Ulva reticulatum and seagrass species, mainly Thalassodendron
species, near a fossil rocky shore. For many years, the presence of the large meadows (beds)
of Halimeda plants in the bay area have remained an open question. Knowing its ecological
importance to the marine environment, and the existing data gap on calcareous algae in the
WIO region, this study was initiated with the following objectives:
(1)
To identify and describe Halimeda species present in the area, study their
distribution, standing biomass, growth, rates of calcification in order to get
estimates of their contribution to the carbonate deposition of the bay.
(2)
Search for mechanisms behind and relations between the photosynthesis and
calcification processes in Halimeda species.
(3)
Examine adaptations in the coralline algae (rhodoliths) explaining their ability
to withstand low tide exposure to high temperatures and excess light that
regularly occur in the area.
8
INTRODUCTION
The worldwide distributed calcareous algae such as the green algae of the genus Halimeda
(Chlorophyta, Bryopsidales) and the coralline red algae (Rhodophyta, Corallinales), has long
been known as main contributors of sand to mud-size carbonate sediments (Drew, 1983;
Payri, 1988; Bosence and Wilson, 2003), primary productivity (Bach, 1979; Multer; 1983)
and provide potential shelter and nursery grounds for a number of invertebrates ((HillisCollinvaux, 1980; Kamenos, et al., 2004) in tropical and subtropical marine environments.
Published information on calcareous algae does indicate that the species are the most diverse
occupying different habitats from intertidal areas to deep waters (Adey, 1998; Basso, 1998;
Aponte and Ballantine, 2001). A group of coralline algae known as rhodoliths form large
dense beds usually referred to as “rhodolith beds” or “maerl grounds” with wide ecological
importance (Maudsley, 1990; Chisholm, 2000). Payri, (1988) reported a number of results
from the previous studies on calcium carbonate production including those from Moorea
reefs, where coralline algae (Porolithon onkoides and Hydrolithon reinboldii) produced
between 26-162 g CaCO3 m-2 y-1. Similarly, Kennedy, et al., (2002) reported 10-53%
production of sand-sized carbonate sediments from coralline algae around Lord Howe Island
and Balls Pyramid, Southwest Pacific. Potin, et al., (1990) reported 876 g CaCO3 m-2y-1
production from Lithothamnion corallioides in the bay of Brest, France. Bosence and Wilson,
(2003) reported calcium carbonate production between 30-250 g CaCO3 m-2y-1 in western
Island and between 895-1423 g CaCO3 m-2y-1 from Norway. However, growth of rhodoliths is
generally slow and growth rate estimates are rare, even those reported so far, are based on
methods which are questionable (Foster, 2001). For example, Bosence and Wilson, (2003)
reported growth rate of 0.5-1.5 mm y-1 from northern east Atlantic. Similarly, Foster (2001)
gave a list of growth measurements results of rhodoliths from several authors who used
14
C
dating method, and commented that the results were questionable.
Halimeda species are capable of producing extensive biohermal accumulations (Davies
and Marshall, 1985) and meadows (Hillis-Collinvaux, et al., 1998), and can be evaluated in
the field using different methods for growth (Bach, 1979; Multer, 1983; Ballesteros, 1991)
and sediment generation (Wefer, 1980; Drew and Abel, 1985; Payri, 1988). They are known
to contribute significantly to the flux of carbon and carbonate sediments in the marine
environment (Wefer, 1980; Braga, et al., 1996). For example, in the Great Barrier Reef, a
2,000 km2 (1,250 sq. mile) area covered with coarse gravel from 10-15m (33-50 ft) deep, was
found to be primarily Halimeda fragments with vast areas comprising as much as 98% algal
9
deposits (Drew, 1983). The same study on the Great Barrier Reef reported huge meadows of
Halimeda produced up to 2 kg calcium carbonate per m2 every year (Drew and Abel, 1985). A
number of studies on carbonate sediment generation (Bach, 1979; Wefer, 1980; Multer, 1988;
Payri, 1988; Ballesteros, 1991) have reported significant results. Nevertheless, field
measurements of growth rates of individual Halimeda species are still limited, possibly due to
difficulties in measuring growth in a plant that grows by unpredictable spurts and varies in
percentage CaCO3 with age (Hillis-Collinvaux, 1980; Drew and Abel, 1985), and differs in
growth rate by species (Hillis-Collinvaux, 1980), and possibly with depth (Böhm, 1973). Such
variables combined with disasters such as storm damage to experimental sites (Merten, 1971),
mechanical damage through human activities (this study) and sometimes the patch
distribution of Halimeda (Drew, 1983) discourage attempts to evaluate production rates
quantitatively.
Study Area
Chwaka bay (Fig. 1) is a relatively shallow tropical lagoon (mean depth 3.2 m) located in
a tropical climate stretching 34 km on the east coast of Unguja Island in Zanzibar, between
39o22’ to 39o30’E and 6o8’ to 6o15’ S (Cederlöf, et al. 1995). The lagoon is mainly
characterized by seagrass beds, macroalgae, some remains of hard corals and mangroves. The
bay experiences semidiurnal patterns of tides with ebb currents are stronger than the flood
currents, and is a potential source of important biological and commercial activities
(Wolanski, 1989; Tobison, et al., 1998). The seabed is broadly influenced by a wide network
of channels with the water currents predominantly forced in a north-south direction. The
vegetative assemblages found in the eastern and south-eastern parts shows distribution with
irregular meadows of different seagrass communities dominated by Cymodocea serrulata, C.
rotundata, Thalassodendron ciliatum, T. hemprichii, Enhalus species and macroalgae, mainly
Halimeda species. Extensive mangrove forests fringes along the Mapopwe creek, in the east,
south, southwest and southeast shoreline (Wolanski, 1989; Mohammed, 1998). The middle,
east and south-west parts of the bay is characterised by a wide continuous seagrass meadows
partly interspersed with a great amount of the macroalgae mainly Halimeda species, and other
such as Sargassum, Ulva and Gracilaria species.
10
Tanzania
Tanzania
Mainland
Zanzibar Is.
5 58 18 S
Zanzibar Town
Chwaka
6 25 26 S
N
0
Km
20
39 05 26 E
39 32 34 E
Fig. 1: Map of Africa (Top right) showing the position of Tanzania in Africa and the position of Zanzibar from
Tanzania mainland with enlarged map of Zanzibar (below) showing the position of Chwaka bay in the East coast
where this study was conducted.
Algae
Traditionally, the term algae refer to macroscopic, filamentous and multicellular marine
red, green and brown halophytes (plants lacking true roots, stems and leaves). Unlike
terrestrial plants, algae are photosynthetic organisms with single reproductive structure,
lacking vascular systems and their body is referred to as thallus (Jaasund, 1976). They contain
a variety of carotenoids depending on the taxonomic group, and all contain chlorophyll a, and
some have chlorophyll b or c (Falkowski and Raven, 1997). Most algae are found in aquatic
environments (freshwater to marine), but some can be found in other places such as rocks,
deserts, soils snow and hot springs. Classification of algae was formerly based on colour,
where algal groups were given names such as “red”, “brown” or “green”. However, at present
11
and according to Van den Hoek, et al. (1995), classification is entirely based on specific
characteristics such as cellwall composition, photosynthetic pigments, storage products,
morphology and ultra structure. Nevertheless, the names of the divisions and classes still
reflect the colour of the main pigments; for example Chlorophyta (green algae), Rhodophyta
(red algae), Phaeophyta (brown algae) and Bacillariophyceae (diatoms).
Calcifying algae
This is a group of algae with an ability to deposit CaCO3 around or within the algal thalli
(Borowitzka, 1982). Among the Rhodophyta, calcification occurs in the Corallinales
(Coralline algae), some members of Bangiales, Gigartinaceae and Squamariceae (Borowitzka,
1982; Kangwe, 1999). Members of the Corallinaceae are both the most abundant and bestknown calcified red algae (Littler, 1972; Borowitzka, 1982). In marine green algae such as the
Halimeda species and the calcifying brown algae Padina, as well as the red algae not
belonging to the family Corallinaceae, deposition of CaCO3 form is invariably extracellular
aragonite, largely in the shape of needle-like crystals (Borowitzka, et al., 1974; Borowitzka,
1981; Braga, et al., 1996). The aragonite form of CaCO3 isomorph in Halimeda is
orthorhombic, whereas in most coralline algae (red algae) the calcite form of CaCO3 is the
rhombohedral carbonate mineral (Milliman, et al., 1974). The Ca2+ in calcite can be replaced
by cations of smaller radius (Mg, Fe, Zn, Cd), while aragonite accepts cations of larger radius
than Ca2+ (such as Pb, Ba, Sr) (Borowitzka, 1977; Kangwe, 1999). High concentrations of
MgCO3 are an indication of calcite, where aragonite is often characterized by relatively large
amounts of SrCO3 (Littler, 1972). Recent studies on calcification process in calcareous algae
are still rare. Most of the studies focus on coral reef ecosystems which are the most striking
example of calcifying ecosystems (Gattuso, et al., 1999).
The genus Halimeda
The green algae of the genus Halimeda belongs to the phylum Chlorophyta, order
Bryopsidales. The family Byopsidaceae/Halimedaceae where Halimeda belongs is along with
their close relatives Udotea and Penicillus, commonly know as Shaving Brush algae (HillisCollinvaux, 1980). A typical Halimeda plant is a flexible string of flattened jointed leaf-like
structures often referred to as segments (Fig. 2). The plant is sometimes called the “money
plants” as it looks somewhat like small coins (Vroom, et al., 2003). Each 'coin' or segment is
12
hard because it is impregnated with calcium carbonate (Drew and Abel, 1985), connected to
its neighbours by a thin strand known as genicula, which gives the plant its flexibility (Multer,
1988). Growth is attained by additional of new segments at branch tips which rapidly achieve
full size before calcification begins (Batch, 1979; Drew, 1983).
Fig. 2: Halimeda macroloba (large plant) and H. opuntia (smaller plant beneath) in an aquarium tank. The
algae were collected from Chwaka bay during field work for use in laboratory experiments. The flattened jointed
leaf-like structures is obvious. New segments can appear at the top of each segment.
Halimeda species inhabit a range of habitats from intertidal zone (this study paper I and
part of paper II) usually mixed together with seagrasses and other macroalgae, in sandy floors
of lagoons and extend to deeper reef slopes (Drew, 1983; Littler, et al., 1985). They are
known to be among the deepest living photosynthetic organisms, found at depths up to 130 m
(Littler, et al., 1986). These algae are somewhat different in that they are both coenocytic
(lacks cross-walls in its component siphons) and calcareous (composed mainly of CaCO3)
(Payri, 1988). The coenocytic thallus, suggest that the genus and other members of
Bryopsidales have distinctive branches on the algal evolutionary tree (Hillis, 2001). While a
13
normal plant cell is tiny, enclosed in a cell wall and contains one nucleus with the genetic
material, coenocytic plants can be thought of as a single giant cell with multiple nuclei (Payri,
1988). About 14 species of Halimeda have been described for the tropical and subtropical
western Atlantic (Hillis-Colinvaux, 1980; Wynne, 1986), and several others have been
described for modern reefs based on morphological properties of the thallus (Drew and Abel,
1988; Hillis, et al., 1998). They provide shelter and sometimes food to a number of reef
animals, and as a colonizer, facilitates the restoration of damaged or eroded reefs (Bach, 1979;
Hillis, et al., 1998). The calcareous nature of Halimeda and the ability to synthesize noxious
and potentially toxic secondary metabolites makes them less appetizing meal to grazing fish
such as surgeon and parrot fishes than more succulent algae (Paul and Fenical, 1983; Hay, et
al., 1988), thus making them protected from herbivory feeders such as parrot fishes. The
compounds halimedatrial and halimedatetraacetate are diterpenoid compounds that appear to
give Halimeda an extremely noxious taste and could prove toxic in large quantities (Paul and
vanAlstyne 1988b). Younger segments have the highest concentration of these compounds,
while older segments are protected by heavier calcification that make them rich of CaCO3 in
the algal wall which makes the plant less tasty to herbivores (Hay, et al., 1988; Braga, et al.,
1996). Death of Halimeda tissues disintegrate into fine, white calcium carbonate particles
(Milliman, 1977; Bach, 1979). The white sandy beaches of some coral atolls may be made up
mostly of Halimeda and coralline alga (rhodoliths) remains (Grall and Hall-Spencer, 2003).
The genus Halimeda is an important element of tropical reefs (Hillis-Collinvaux, 1980), a
contributor of sand and carbonated sediment in tropical reefs since mid-Jurassic to the
Holocene period (Hillis, 2001). The greatest pre-Cenozoic species diversity was achieved
during the latter part of the cretaceous (Flügel, 1988; Kooistra, et al., 2002). According to
Hillis (2001), the long paleohistory is capped by an apparent burst of speciation associated
with the Holocene (Fig. 3), and at least three time-periods during the ca. 260 million years
ago of Halimeda history are likely to have had major impacts on evolution of the genus; (1)
Cretaceous-Tertiary boundary events; (2) closing of the circumtropical Tethyan seaway with
associated Messinian crisis; and (3) final closure of the Panama seaway. However, little
information can be obtained from the recorded paleohistory of Halimeda, and the first
phylogenetic data (evidence) of the genus came from analysis of the 18s DNA sequence, and
phylogenetic trees were presented to indicate geographical distribution and separation of the
rhipsalian species into Atlantic and Pacific clades (Mankiewicz, 1988; Flügel, 1988).
14
40
35
%
K T
Number of species
30
25
Messinian
crisis
20
15
10
5
H
ol
o
ce
ne
ce
ne
Pl
ei
to
ce
ne
Pl
io
e
M
io
ce
n
O
li g
oc
en
e
Eo
ce
ne
La
te
Cr
et
ac
eo
us
Pa
le
oc
en
e
ta
ce
ou
s
Ea
rly
Cr
e
Pe
rm
ia
n
0
Geological time-scale
Fig.3. Diversity of Halimeda species in geological time-scale determined from the fossil data. The figure
suggests peaks of diversity during the time period represented by late cretaceous to Eocene, that is from ca. the
last 30 million years ago of the Mesozoic through approximately the first half of the Cenozoic. The comparative
species richness is followed by seemingly very low diversity before an apparent burst of speciation in the
Holocene (Source: Hillis, 2001).
Reproduction in Halimeda
Halimeda and other closely related members of Bryopsidales have an ability to reproduce
both sexually and asexually. Sexual reproduction is rarely seen in Halimeda because it is a
short lived phenomenon, and has recently been described for one species from direct
observation in the field (Clifton, 1997). The ability of Halimeda to propagate asexually via
vegetative fragmentation has been mentioned as one of the reason on why it’s abundant on
coral reefs (Walters, et al., 2002). Asexual propagation occurs through vegetative
fragmentation when detached live portions of individuals survive and continuous to grow. In
the marine environment, fragmentation via fission may be; (1) an endogenous (Yamashiro
and Nishihira, 1998) (2) as a result of exogenous processes, such as predation or physical
disturbance events (Walters and Smith, 1994). The advantage of fragmentation over sexual
reproduction includes extension of the distribution of genets and species, increase in the
abundance of the organism and individual biomass, and colonization of areas where sexual
15
propagules are unable to settle or high rate of early post settlement mortality. Nevertheless,
recent research shows that fecundity of the organism is reduced especially when the fragments
are dispersed in an unfavourable habitat (Smith and Hughes, 1999). Therefore, the costs of
fragmentation outweigh the benefits for some marine organisms and this may be related to the
organism’s potential to successfully sexually reproduce.
Sexual reproduction by many members of the Bryopsidales, including Halimeda, is
holocarpic, with dioecious individuals releasing gametes all at once and then dying within
hours (Clifton and Clifton, 1999), and the thallus completely disintegrates after spawning.
The spawning process is initiated after sunset (Drew and Abel, 1988). The simple life-history
of Halimeda (Fig. 4) is illustrated by a free-living phase that reproduces sexually is seen to
have a dynamic, asexual, fragmentation component that allows for long term viability and reestablishment of fragments. Recent observations have shown that sexual reproduction in
Halimeda to some extent is synchronised (Hay, 1997), where many individual in a population
may become fertile within a period of only few days, or sometimes on the same day.
Fig. 4: Sexual reproduction and the general life cycle of Halimeda species. Halimeda can reproduce very
successfully sexually and through vegetative propagation which enables copies of the same plant to be produced.
Many species can produce filaments which can grow more than 20 cm long, spread laterally through the
substrate, and then push up to form new segments. Eventually the physical connections between the young and
parent thallus are lost (Source: http://www.aims.gov.au).
16
Rhodoliths
The term rhodoliths (or maerl) is used to define nodules and detached branched growth
with a nodular form composed primarily of coralline algae (Basso, 1998), that occurs not
confined to a particular benthic zone. They belong to a group known as coralline red algae
that deposit CaCO3 within their cell walls
to form hard structures that closely
resemble
maerls
or
pearls,
which
accumulate to form large beds and have
been found throughout the world’s oceans
(Wilson, et al., 2004). They are known as
rhodoliths when the corallines are made
up to more than 50% of the nodule
(Basso, 1998) or composed entirely of
non-geniculate coralline algae (Foster,
2001). They are critical habitats for many species including fishes, clams and true corals
(Basso, 1998; Kangwe, 1999). However, unlike corals, rhodoliths do not attach themselves to
the rocky seabed, rather they drift along the seafloor until they grow heavy enough to settle
and form brightly colour beds (Basso, 1998). The most difference between rhodoliths and
corals is that, the corals filter plankton and other organisms from water for food, whereas
rhodoliths produce energy through photosynthesis (Borowitzka, 1981).
Rhodoliths grounds (also known as Maerl grounds) are composed of loose-lying nongeniculate coralline red algae (Foster, 2001), and are mostly found in areas characterised by
high water movements (wave action) in the photic zone (Kamenos, et al., 2004). Maerl
grounds vary in size and are dense accumulations of unattached coralline algae and occur
throughout the world oceans (Woelkerling, 1988). They serve two main functions (1)
production of calcareous and carbonate sediments important for reef building and corals (2)
with their structural complexity provide relatively stable microhabitats which are important
shelters and nursery grounds for the increasing number of refuges from predators (Grall and
Hall-Spencer, 2003). Maerl grounds have been found to fulfil the density and refuge
prerequisites of a nursery area for a number of invertebrates and vertebrates (Kamenos, et al.,
2004). Maerl grounds are important in sustainable fisheries, providing nursery grounds for
commercial fish species and shellfishes. Fragments and hard substrate may originate in the
17
bed or be broken from nearby reefs and transported to the site of growth, where new rhodolith
beds established in this way. Apart from their ecological importance rhodoliths (maerl)
contains resources economic importance which can be extracted and used primarily as soil
conditioners instead of ground limestone, and as a water filtration and conditioning agent in
Europe (Foster, 2001). For example, in France, maerl beds in Brittany represent the largest
resources (Grall and Hall-Spencer, 2003) and accounts for 80% of the total 500,000 t
extracted annually. This enables the maerl industry provide hundreds of local jobs to the
people (Bosence and Wilson, 2003). However, due to their slow growth rate between 0.5-1.5
mm per year (Bosence and Wilson, 2003), maerl beds are considered as non-renewable
resources. To save the maerl from being over-exploited, some efforts are being made by some
countries in Europe (example France) by introducing laws through their marine conservation
programmes which include maerl grounds for conservation (Grall and Hall-Spencer, 2003).
Photosynthesis and sources of inorganic carbon in aquatic environments
In both terrestrial and marine plants, photosynthesis is usually regarded as the main
indicator of performance, adaptation and physiological status. The processes produces energy
using certain wavelengths of light, involving two photosystems, PSI and PSII which are
mostly active at 680 and 700nm wavelengths. Other wavelengths are also peaks in the action
spectrum for photosynthesis. Autotrophs use CO2 and energy from the sunlight to synthesize
organic molecules (such as glucose). Plants are autotrophs, which means they are able to
synthesize food directly using carbon dioxide gas, water and light to produce sugars and
oxygen gas. For instance, the production of glucose can be simply represented in an overall
chemical equation;
12H2O + 6CO2 + Light → C6H12O6 + 6O2 + 6H2O
Even though, this equation may appear simple, but it is actually a summary of very
complex processes (Falkowski and Raven, 1997). The glucose is variously used to form other
organic compounds, such as the building material cellulose, or it may be used as a fuel to
drive other physiological processes of the plant. This takes place through respiration found in
both animals and plants. The chlorophylls in plants absorb light energy that drives the process
of photosynthesis. Contrary to the terrestrial environment where there is a plenty of CO2
available in the atmosphere for the plants to use in photosynthesis, submerged plants
experience problems of inorganic carbon acquisition in photosynthesis (Hellblom and Björk,
18
1999). This is due to slow diffusion rate of CO2 in aquatic environment which makes CO2 less
available to aquatic macrophytes (Beer, et al., 2002). Thus, depending on the pH of the
aquatic media, four forms of inorganic carbon exist; CO2, HCO3- CO32- and H2CO3. At low
pH 6.15-6.5, CO2 is abundant such that it can drive high photosynthetic rates by sole diffusion
into the site of carbon fixation from the bulk waters (Borowitzka and Larkum, 1976d). At pH
8.2 sea water, more than 90% of the inorganic carbon available in the aquatic environment is
in the form of HCO3- (Hellblom and Björk, 1999), furthermore the diffusion of CO2 in water
is drastically slower than in air (Falkowski and Raven, 1997; Hellblom, 2002).
At the normal pH ranges of sea water (8.1-8.2), poor CO2 availability is thus quite often
limiting productivity in marine plants. To solve the problem of CO2 limitation, aquatic plants
have developed mechanisms (modes) for using HCO3- available as the main source of
inorganic carbon in photosynthesis. The following mechanisms for inorganic carbon
acquisition in aquatic environment have been described and reported (1) direct diffusion of
CO2 into the cell (2) spontaneous dehydration of HCO3- to CO2 due to locally elevation of H+
concentration outside the plasma membrane (acid zones)(Hellblom, et al., 2001). This
mechanism is based on natural processes, where H+ are pumped from the cytosol resulting
into accumulation of protons outside the plasma membrane and consequently lowering the pH
which favours more dehydration of HCO3- to CO2, followed by its diffusion into the cell
(Hellblom, et al., 2001; Klenell, et al., 2004). The drawbacks of this natural process
(protogenic mechanism) is that, it can be negatively inhibited by the presence of biological
inhibitors such as TRIS buffers which competes for uptake of protons (Hellblom, 2002) (3)
Extracellular dehydration of HCO3- catalysed by carbonic anhydrase (CA), an enzyme
localized outside or within the plasma membrane (Axelsson, et al., 2000; Hellblom, et al.,
2001), which speeds the conversion of HCO3- to CO2 under normal pH 8.0-8.2 (4) HCO3- can
be actively transported across the membrane by protogenic HCO3- uptake through a symport
H+/HCO3- or co-transport (Price and Badger, 1985; Hellblom, et al.. 2001). This mechanism is
closely similar to the second mechanism, and both can be inhibited if the acid zones are
dissipated by biological buffer (5) CA-catalysed HCO3- dehydration within the acid zones.
This mechanism takes advantage of the protons excreted within the acid zones creating a
more favourable condition for CO2 conversion. The different forms of inorganic species in sea
water are produced when carbon dioxide enters the aquatic environment and reacts water in
the following sequence;
19
CO2 + H2O ↔ H2CO3 ↔ H+ +HCO3- ↔ 2H+ + CO32-
(1)
OH- + CO2 ↔ HCO3-
(2)
Equation 1 above starts when CO2 from the atmosphere enters the aquatic environment
and reacts with water to form a weak carbonic acid which is less stable and dissociates to
HCO3- and H+, which dissociates further to form CO32- when pH increases. At higher pH
above 8.2 the OH- can combine with available CO2 to form HCO3- (equation 2). In general,
sea water at pH 8.2 contains a smaller proportion of CO2 (10 μM ) and more than 95% (about
2.1 mM) of total Ci is in the form of HCO3- (Falkowski and Raven, 1997).
Algal calcification
The word calcification refers to the precipitation of CaCO3 around or within the algal
thalli (Borowitzka, 1982) of calcifying algae such as those in the Rhodophyta, brown algae
(Padina) and Chlorophyta (Halimeda). Calcification process in Halimeda is active in light
and lower in the dark (Borowitzka and Larkum, 1976b). Respiration reduces calcification,
probably due to lowering of pH in the intercellular space as a result of CO2 production
(Borowitzka, 1977). Plant metabolism may stimulate calcification by increasing the local
concentration of Ca2+ and/or CO32- ions, or by removing inhibitors of CaCO3 precipitation
such as phosphates which are known as crystal poisons (Simkiss, 1964).
To explain calcification in Halimeda, a researcher needs to take into consideration the
specialized morphology of the thallus of these algae (Borowitzka and Larkum, 1976c).
Calcification takes place when photosynthesis is active and the intercellular space (ICS:
which represents a large portion of exchangeable calcium) must be separated from the
external medium by loose peripheral utricles (Borowitzka and Larkum, 1976c). This
morphology means that the supply of ions to the ICS must be by diffusion through the outer
cuticle and the cell walls of the appressed utricles. Multer, (1988) studying the ultra-structure
of H. incrassata and H. monile found a unique aspect of development of large intercellular
spaces (ICS) in which CaCO3 in form of aragonite was deposited. A typical equation of the
process can be represented as follows;
CO2 + H2O ↔ HCO3- + H+ ↔ CO3- + 2H+
(1)
20
CO32- + Ca2+ ↔ CaCO3↓
(2)
2HCO3- + Ca2+ ↔ CaCO3↓ + CO2 + H2O
(3)
Equation 1 above shows the hydration of CO2 in ocean waters in equilibrium with air
levels of CO2, resulting in different forms of carbon and proportions of the different carbon
species of the equilibrium reactions are dependent on pH of the media (Axelsson and
Uusitalo, 1988). At lower pH a large proportion of inorganic carbon is present in the form of
CO2 and the reactions to the left are favoured, whereas at higher pH like pH 8-9, the majority
of carbon is present as bicarbonate and carbonate (Hellblom, 2002). The third one is often
used to emphasize that under normal pH conditions of any natural waters (pH 8.1-8.2), the
HCO3- ions largely dominate over the CO32- ions (Johnston, et al., 1992; Hellblom, 2002).
Photosynthetic CO2 uptake from the intercellular spaces increases intercellular pH where
CO32- in the presence of Ca2+ combines to form CaCO3- which is deposited as aragonite, thus
facilitating calcification process (Borowitzka, 1982). However, this mechanism for
calcification in Halimeda is not applicable to all aragonite depositors such as the brown alga
Padina, where there are no intercellular spaces and aragonite is precipitated in concentric
bands on the outer surface of the thallus (Lobban and Harrison, 1994). Moreover, there are
other seaweeds with apparently suitable morphology that do not calcify (example,
Enteromorpha) (Borowitzka, 1982).
Despite calcification being one of the important structural processes in the oceans, its
mechanism in algae is not fully understood. It is however well known that calcification is
directly proportional to photosynthetic rates and is stimulated by light (Borowitzka, 1977),
and that, calcification rate is highest in the young tissues (Lobban and Harrison, 1994).
Moreover, taking in consideration the type of CaCO3 deposited, localization and organization
of the cell wall matrix, it appears that the calcification process in algae involves more than
one mechanism (Borowitzka, 1977), and crystal formation requires two steps; crystal
nucleation and crystal growth. Nucleation is the major rate limiting step for the precipitation
of CaCO3 and can be used to explain why other algae do not calcify (Borowitzka, 1982).
21
The link between algal photosynthesis and calcification
Knowledge of the source and forms of inorganic carbon for photosynthesis in aquatic
environment is important for understanding calcification mechanisms in algae. Previous
studies on photosynthesis and calcification processes have shown that the two processes are
coupled in a certain way (Borowitzka and Larkum, 1976b, c, d; Borowitzka, 1977; Pentecost,
1978; Gattuso, et al., 1999; Borowitzka, 1982; Marshall and Clode, 2002). Light stimulated
algal calcification involves a rise in CO32- as a result of CO2 uptake during photosynthesis or
due to alkalization of the medium due to OH- extrusion from the cell after HCO3- uptake
(Borowitzka and Larkum, 1976b). Similarly, it has been suggested that calcification enhances
photosynthesis by providing protons that convert seawater HCO3- to CO2 and H2O, thereby
supplying some of CO2 for photosynthesis (Borowitzka and Larkum, 1976c; McConaughey
and Whelan, 1997). The rhythm and nature of calcification processes may be estimated in
different ways, but the measurement of TA changes in seawater is considered as the most
convenient in short time duration experiments (Smith and Key, 1975; Chisholm and Gattuso,
1991).
In the course of development in research on photosynthesis and calcification, Borowitzka
(1977) put forward a number theory explaining the link between calcification and
photosynthesis mechanisms. The widely accepted theories include; (1) CO2 usage theory:
This suggest that, photosynthetic CO2 uptake spontaneously generated from HCO3- may
increase extracellular pH (due to alkalization of the medium caused by OH- extrusion from
the cell after HCO3- conversion to CO2) high enough to elevate the concentration of CO32which leads to extracellular precipitation of CaCO3 in the presence of Ca2+ ions. However,
this process can be inhibited if the acid zones are dissipated by a biological buffer such as
TRIS or AZ (Price and Badger, 1985; Hellblom, et al., 2001), or if there is a limited
spontaneous generation of CO2 from HCO3- (Fig. 5) (2) HCO3- usage theory: Suggest that, by
the aid of the enzyme carbonic anhydrase, the photosynthesizing algae which uses HCO3- as a
source of carbon, may extrude OH- to specific zones outside the plasmalemma which will
favour the precipitation of CaCO3. (3) Organic matrix: The presence of charged Calcium
binding mucilage (polysaccharide complexes) in the cell walls (Borowitzka and Larkum,
1976b) acts as nucleation sites for the Ca2+ crystals. The form of CaCO3 to be deposited is
suggested to be determined by the nature of polysaccharide of the relevant alga (Borowitzka,
1977). Non-calcifying algae may have the same cell wall organisation, but it is not known
why they do not calcify (Borowitzka, 1982).
22
Despite such conceptual agreement on the link between photosynthesis and calcification,
Yamashiro (1995) showed that bisphosphonate (as a crystal poison) reduced 14C incorporation
in into the skeleton (CaCO3 deposition) but not into the tissues (photosynthesis) of
zooxanthellate coral, and concluded that calcification is not necessary for photosynthesis.
Similarly, Gattuso, et al., (2000) showed that artificial seawater with a low calcium
concentration lowered calcification rate but did not reduce the production of photosynthetic
oxygen and concluded that “calcification is not a significant source of photosynthetic CO2”.
CELL
CO2
Respiration
Photosynthesis
HCO3-
CO2 + OH-
OH-
HCO3- OHCO2
CO2 + H2O
CO2 + H2O
HCO3- + H+
HCO3- + H+
CO32- + H+
CO32- + H+
Ca2+
Ca2+
CaCO3
SEA WATER
INTERCELLULAR
SPACE
Fig. 5: Schematic representation of possible mechanisms of Ci uptake and postulated ion fluxes for CaCO3
precipitation in Halimeda species during photosynthesis and calcification processes in seawater. Passage of ions
from seawater to the intercellular space is by diffusion through the cell wall of appressed utricles. CO2 for
photosynthesis enters the cell by diffusion from both the external medium and from the intercellular spaces
(ICS), and CO2 produced during respiration diffuses out of the cell. HCO3- enters the cell by periplasmic CAmediated dehydration or mediated by H+-ATPase. After dissociation of the HCO3- the OH- may leave the cell
possibly in much localized region [Modified from Borowitzka and Larkum, 1976c].
23
COMMENTS ON MATERIALS AND METHODS
The use of Satellite Remote Sensing in assessing vegetation cover
Field work
Seasonal and spatial distribution of bottom vegetation cover (ground-truthing) was
assessed visually using SCUBA facilities. The area was divided into 221 sampling sites, 500
m apart from each positioned using a GPS. Ten 0.25 m2 metal frames were randomly placed
at each site, followed by assessment of vegetation cover within the frames. The sites were
seasonally assessed in December 2000, March, June and September 2001. The field work for
2002 on SAV coverage (structural complexity, shoot density, above-ground biomass and
canopy height) mainly focused on six selected sites representing homogenous and dense
meadows within the bay using methods described in paper I. The aim was to use the data to
describe the most dominant seagrass communities, and compare well-quantified seagrass
habitats with spectral signatures derived from Satellite Remote Sensing. Apart from analysis
of seasonal variations between assessment periods, a correlation analysis was made between
total submerged aquatic vegetation (SAV) coverage between years using Landsat ETM+
images available. A regression analysis was applied between percentage cover field data
(ground truthing) obtained in September 2001 and the digital spectral values from Landsat
ETM+ scene taken in the same month. However, due to the effect of cloud cover, only 107
sites out of 221 were used in this analysis. A field work conducted in 2004 aimed at verifying
the positions and extensions of the Remote Sensing mapped major habitats. In addition, an
interview (discussions) was held between local people and fishermen of Chwaka village to
obtain information on the cause of the observed changes in vegetation cover in certain parts of
the bay.
Image analysis:
The digital spectral values from September 2001 Landsat ETM+ image were compared
with vegetation coverage obtained from field work on the same month. The satellite sensors
used for this study were (1) Thematic Mapper (TM) on Landsat 5 and (2) Enhanced Thematic
Mapper (ETM+) on Landsat 7. The sensors had a resolution of 30 x 30 meters (which was
appropriate for this study) for six identical matching spectral bands, including the blue band
which is important for water penetration. The TM data is available since 1982; while the
ETM+ data is available since 1999. These data creates a possibility of mapping the coastal
environment over a longer period. Using a computer program analysis, the digital spectral
24
values from the September 2001 Landsat ETM+ image were compared to the vegetation
coverage (ground-truthing) assessed by the field surveys in that month. This was followed by
computer-based unsupervised classification (Mather, 2004) image analysis (after creating
masks in the two images to exclude features such as land, deep ocean, clouds so that the
analysis focuses on the features of interest) to give a clear relationship between the sensitivity
of the satellite data spectral response in relation to vegetation coverage on the bottom of the
bay. The visible bands of the satellite image from September 2001 were used. Data analysis
also involved the use of two geometrically corrected satellite images (Landsat TM from 27
January 1987 and Landsat ETM+ from 30 January 2003), which were used to create a change
detection map. Several other procedures were applied during image analysis including the use
of the visible wavelengths (blue, green and red) due to their ability to penetrate water, a crude
atmospheric correction (which was done by applying “dark pixel subtraction”), selection of
training areas for the two classes (SAV and un-vegetated areas) and making a supervised
maximum-likelihood classification, where the acquired map was compared to the outcome
from the field surveys. Several Landsat images were available from 1986 to 2003, which were
used to illustrate the general vegetation changes within that period by performing correlation
analysis of digital radiance values from 0-255 for pairs of satellite images.
Contributions from Halimeda species in the bay
Species composition, growth, standing biomass and distribution of Halimeda including
their contribution to carbonate production (paper II) and associated parameters (tissue nutrient
content, decomposition, carbonate production, in situ photosynthesis and calcification) were
assessed between the year 2000 and 2003 using methods previously used by other authors
(Smith and Kinsey, 1978; Drew and Abel 1988; Payri, 1988; Ballestros, 1991; Ochieng and
Erftemeijer; 1999). These methods have proved to yield significant results in the past.
Identification of species composition in the area was done primarily using classical
morphological descriptions provided by Jaasund (1976) and Hillis-Collinvaux, (1980),
followed by final identification and confirmation by a taxonomist. Growth was determined
using tagging method (Drew, 1983; Ballesteros, 1991) at two sites differing in ecological
characteristics (substrate type, extent of exposure to air during lowest spring tides of the day
and inshore vs. offshore). This was important for comparison to see if there is a significant
difference in growth rate between the two sites, and between seasons (dry and wet seasons).
Currently, most of the biomass studies on Halimeda have been reported from outside the
Western Indian Ocean (Bach, 1979; Drew, 1983; Drew and Abel, 1985; Garrigue, 1991;
25
Payri, 1988). Thus, standing biomass was assessed for six months at five sites using method
(Bach, 1979; Ballesteros, 1991), where ten, 0.25 m2 quadrants were used. During each visit,
the quadrants were thrown in random at each site, followed by quantification of the above
ground biomass from each quadrant, dried to constant weight and presented as gdw m-2. Like
for the growth assessment, the aim was to investigate if there is a difference in standing
biomass between the sites during rain and dry seasons. Carbonate production assessment was
conducted using acid leaching method to get estimates on how much carbonate materials are
produced by Halimeda species in tissues, and extrapolate (using standing biomass data and in
situ calcification values) into carbonate production m-2, and compare with the previous studies
from other parts of the world’s oceans (Böhm, 1973; Wefer, 1980; Drew, 1983; Multer, 1988;
Payri, 1988). It was also of added advantage to assess tissue nutrient content (C, N and C/N
ratio) on Halimeda materials collected from inshore and offshore, so as to study if there is
nutrient gradient between inshore and offshore habitats (Hemminga, et al., 1994; Mohammed,
1998), and between seasons (Boto and Bunt, 1981; Ballesteros, 1991). Analysis of dry
Halimeda opuntia materials collected monthly was conducted in the Department of Systems
Ecology, Stockholm University, Sweden. Duplicates of about 3-3.8 mg of dry and grounded
algal materials from each site were analyzed for C, N and C/N ratio using elemental analyzer
(LECO CHNS-932) and expressed in % C, N content (dry weight) and the C/N ratio was
calculated. Decomposition of Halimeda materials is an important phenomenon regarding
sediment generation for development of reefs and tropical lagoons (Milliman, 1977; Drew,
1983). This was conducted at two sites for 56 days using litter bag experiments (Ochieng and
Erftemeijer; 1999), with subsequent deployment of 6 litter bags from each site for analysis
after every 8 days. The aim was to investigate decomposition rates (time in days) it takes for a
given amount of Halimeda materials to decompose into sediments. In situ calcification was
determined using total alkalinity (TA) method (Anderson and Robinson, 1946; Smith, 1973;
Smith and Kinsey, 1978) as described in paper II. The method was appropriate to this
investigation, where dark and light bottles were incubated in situ for 4 hours, followed by pH
measurements in the laboratory, and calculations on calcification and carbonate production
were made.
Metabolic inhibitors of inorganic carbon uptake
The effect of biological inhibitors or varying pH (paper III) on inorganic carbon uptake
into photosynthesis and calcification was conducted in the laboratory by total alkalinity
method during dark and light incubations using a newly developed device, the Titration
26
manager (TM865, Radiometer analytical, Denmark) equipped with an automatic sample
changer (SAC80 Radiometer analytical Denmark). The TIM865 can be programmed and run
titration process automatically to completion. Measurements of electron transport rate (ETR)
were measured and recorded using a well known device, the Pulse Amplitude Modulation
fluorometer (the diving PAM, Walz, Germany). A Clark type electrode was used for
measuring dissolved oxygen with a temperature sensor (Oxi 323 electrode connected to a
multi 340i meter, WTW Germany) which was immersed into a closed incubation cylinder,
through a hole in the lid, sealed with an o-ring. During the experiment the following
parameters were measured and recorded; pH, Total alkalinity, ΔF/Fm´ (or Fv/Fm during
darkness), dissolved oxygen and temperature. The stock solutions of the inhibitors used (100
µM AZ, 10 mM TRIS, 100 µM DCMU, pH 9.0 and 9.8) were prepared and added to the
sample solution to give the required final concentration, and one inhibitor was added at a
time. After addition of the inhibitor, at the start of each experiment, the pH of the
experimental medium was adjusted to 8.2 using NaOH or HCl. Each exposure lasted for 3
hours in alternating dark and light incubations. Calculations of the results involved the
following;
(1) Since for every CaCO3 precipitated, the total alkalinity is lowered by 2, change in
calcification (ΔCcalc) was calculated as change in total alkalinity divide by 2. i.e.
ΔCcalc= ΔmEq/2
(2) Photosynthetic inorganic carbon uptake (ΔCphot) was calculated as the change in total
carbon minus calcification. i.e. ΔCphot=ΔTC-ΔCcalc
(3) The effective quantum yield was calculated as ΔF/Fm’= Fm´-F/Fm´
(4) The maximum quantum yield was calculated as Fv/Fm = Fm-F0/Fm, and the ETR was
calculated as ETR = (ΔF/Fm’) x PAR x AF x 0.5 (Beer, et al., 2001).
(5) The area of the Halimeda samples used was determined using a computer programme
(Canyon), so as to express photosynthesis in μmol C m-2s-1 and dissolved oxygen was
expressed in µmol O2 m-2s-1.
Inhibitor – DCMU
The herbicide DCMU (well know as PSII inhibitor) blocks electron flow from QB- to PQ,
probably by binding at the QB site of D1 protein (Krause and Weis, 1991), so that the electron
is unable to leave QA, the first quinone acceptor. Thus, the binding of a herbicide effectively
27
blocks electron flow from PSII to plastocyanin (PC) towards PSI, and therefore inhibiting
photosynthesis. This has a further effect of a back-transfer of electrons in a fraction of the
centres that are in the state of QAQB (Hodges and Barber, 1986). Therefore, the action of
DCMU is consistent, acting primarily as an inhibitor of photosynthetic electron transport
(Borowitzka and Larkum, 1976d), causing a drop in photosynthesis and consequently
inhibiting calcification process.
Inhibitor – AZ
Acetazolamide (AZ) is a sulphonamide anion which binds to zinc ion of the enzyme CA,
making the enzyme inactive, and thus inhibiting the CA-catalysed conversion of HCO3- to
CO2 and OH- process. This can lead to inhibition of both photosynthesis and calcification
(Stark, et al., 1969; Borowitzka and Larkum, 1976d; Velitchkova and Picorel, 2004).
Inhibitor – TRIS
TRIS buffers are potent inhibitors of H+ dependent HCO3- utilization by their capability to
take up protons (Hellblom and Björk, 1999; Hellblom, 2002; Uku, 2005). They highly
compete for protons uptake and can inhibit other biochemical processes (El Haїkali, et al.,
2004).
In paper IV, since the rhodoliths are found lying on the intertidal area in Chwaka bay,
exposed to high light and temperatures of the midday sun during lowest spring tides of the
day, it was of interest to investigate the tolerance limits of rhodoliths by exposing them to
excess light and temperature, while measuring photosynthesis and calcification rates. This
was determined using a similar experimental set-up as for the above experiments for paper III
on the effect of inhibitors and varying pH, except that the Rhodoliths were first exposed to
25°C and then to a set light intensity (0, 150, 500, 850, or 1200 µmol photons m-2s-1), then to
an elevated temperature (28, 31, 34, 37, 40, 43 or 46°C), and then the temperature was
returned to 25°C again, while all the time keeping the same light. Each exposure lasted for
about 3 hours. Temperature-light combination involved 3-4 experiments in alternating dark
and light incubations and the calculations were done as in paper III. Difficulties were
encountered on calculating the area of the rhodoliths, as they were nearly spherical in shape.
Therefore, we estimated the photosynthetic area as the projected area (since light was
unidirectional), i.e. the cross-section area.
28
RESULTS AND DISCUSSION
I: The use of Satellite Remote Sensing technique has proved to be a useful tool for
mapping and monitoring vegetation cover and distribution in a given area over time (Deysher,
1993; Call, et al., 2003). The technique is quickly replacing the use of conversional
techniques such as aerial photographs and field mapping (Ferguson, et al., 1993; Robbins,
1997), which allow a very limited spatial coverage and could be very expensive in terms of
equipment, time and personnel (Lillesand, et. al., 2004). The results from this study made in
Chwaka bay supports the use of this technique for monitoring seasonal changes in distribution
patterns and cover of SAV as indicated by the strong relationship between the Satellitederived sun reflectance and the in situ quantified SAV percent cover. This suggests that
frequently repeated satellite image acquisition in combination with regional covering is a
great advantage compared to other mapping techniques. Although we found no significant
seasonal variation in SAV during 2000/2001 field work (Fig. 6), but the loss and gain results
obtained in our study from mapping (Fig. 7) were possibly attributed by environmental
parameters such as fluctuations in temperature and salinity, and turbidity as has been reported
from other studies (Robbins and Bell, 2000). It was further learnt that, such changes are
mainly due to mechanical damage by the ongoing human activities in the area including
seaweed farming, intensive fishing activities using modern techniques and shell collection
during lowest spring tides of the day (Tobisson, et al., 1998). These activities might indirectly
result in loss of seagrass habitat, leading to change into bare sediment in some areas (Fig.7, B
and C) while in others into dense SAV (Fig. 7, A). Moreover, severe overgrazing of seagrass
communities due to periods of extreme population densities of sea urchins may bring severe
SAV loss in some place. Personal observation during field work we found aggregations of sea
urchins in several spots within the bay, especially the sea urchin Tripneustes gratilla, known
as an effective seagrass grazer (Alcoverro and Mariani, 2002). In addition, the interviews
conducted between local residents and fishermen revelled similar observations.
Despite the usefulness of Satellite Remote Sensing technique, its use in the aquatic
environment is still at early stages of application (McKenzie, et al., 2001), and is limited to
shallow and clear waters of tropical and temperate ecosystems (Dahdouh-Guebas, et al., 1999;
Lundén and Gullström, 2003). This limitation of its use to shallow depth brings some
difficulties in its application, and due to this problem we found difficulties during data
analysis on discriminating among seagrass species and to separate seagrasses from
macroalgae, which was possibly due to differences in water depth. In addition, the technique
is only useful in areas of cloud-free, thus making difficult to be used in cloud-covered areas.
29
Anyhow, this study has proved that it is possible to monitor changes of seagrass and seaweed
distribution in tropical environments using repeated mapping with satellite remote sensing.
The spectral and spatial resolution acquired by the Landsat TM/ETM+ sensors was
appropriate for this purpose. This type of satellite remote sensing data creates a basis for an
operational and cost-effective monitoring method for conservation and restoration purposes.
Cove rage
100
80
Bare sediment
Other macroalgae
60
%
Halimeda spp.
Seagrass
40
20
0
Dec 2000
Mar 2001
Jun 2001
Sep 2001
Fig. 6: The mean percentage cover of seagrass, Halimeda spp., other macroalgae and bare sediment in Chwaka
Bay during the survey period from December 2000 to September 2001.
Fig 7: Map from satellite image classifications showing the changes in SAV distribution between 1987 and 2003
in Chwaka Bay. The colours represent changed and unchanged areas: yellow = bare sediment to SAV; orange =
SAV to bare sediment; dark green = unchanged SAV; brown = unchanged bare sediment. The letters in the
map refer to the areas specifically described under results.
30
II: In paper II, four species were identified from the study area; H. opuntia (L.), H.
macroloba (Decaisne), H. incrassata (Ellis) and H. tuna (Ellis and Solander). The species are
common inhabitants of tropical and subtropical environments (Jaasund, 1976; HillisColinvaux, 1980; Drew, 1983) and no new species were identified. Environmental parameters
such as substrate type (sandy, soft boom, muddy or rocky), and high water motion had a
major influence on their distribution and segment size. Muddy and soft bottom areas favoured
high density of Halimeda with large segment size compared to rocky, high water motion and
sandy areas (Muzuka, et al., 2001; pers obser). The relative growth rates obtained in this
study was approximately 1 segment tip-1day-1. This is close to the other reported rates outside
the Western Indian region (Bach, 1979; Drew, 1983; Garrigue, 1991; Ballestros, 1991), but
comparisons are difficult because of the methods used. Water temperature was probably one
of the environmental factors that influenced plant growth and development. This is shown by
lower growth obtained during cold period assessment (July-August), where the mean growth
trend shows a reduction in number of segments produced per tip per day towards the colder
period (June-September of every year). Tissue nutrient content (C and N) showed variations
between sites without a specified trend with high C/N ratios above the normal Redfield ratio
for normal growing macrophytes, which indicate that Halimeda species in the area are
nitrogen limited for growth and photosynthesis. Therefore, there is a possibility that nitrogen
was among other factors responsible for the observed variations in growth of individuals.
However, since there is no information available on C/N ratio of calcified algae, it is possible
that this is a normal range for them due to high CaCO3 in their tissues. Salinity fluctuations
and light possibly did not pose major influence on growth of Halimeda species due to high
growth obtained during rain period at site I which was close to the shore and highly
influenced by low salinities during rainfall period due to dilutions by the incoming
freshwaters.
The standing biomass of 1,563 g m-2 obtained in this study is possibly higher than the
previous findings reported elsewhere from other world’s oceans. This is due to the presence
of large meadows (beds) covered by Halimeda species over the entire bay (Muzuka, et al.,
2001; Gullström, et al., in press) which lead to high calcium carbonate production results per
square meter per day. From these findings, it is believed that the bay is annually receiving a
high amount of carbonate materials from Halimeda species. However, despite higher CaCO3
production in the area, lack of data on sediment loss during ebbing represents a budget
imbalance that needs to be fully addressed. There is a possibility that most of the CaCO3
produced is transported by outgoing tides across the “littoral fancy” to deep waters.
31
Decomposition experiment using litter bags indicates that Halimeda materials decompose
faster in the beginning, but slowly in the end. To a larger extent this was due to the reason
that, in the end there is only CaCO3 material left in Halimeda segments (tissues) which
decompose more slowly (Fig. 8). The differences in number of days (103 and 76 for site I and
II respectively) for 50% of materials in the litter bags to decompose into sediments, was
possibly due ecological differences between the two sites. Site I remained submerged even
during lowest spring tides of the day (~1.5 m deep at lowest tide), while site II became
completely exposed to air for long time during lowest spring tides of the day. In addition, the
observed large microbial populations comprising polychaetes, amphipods etc, at site II than at
site I (pers obser), possibly enhanced the decomposition of Halimeda materials at this site.
1.1
Proportion AFDW (g) remaining
1
0.9
Site I
Site II
0.8
R2 = 0.9885
0.7
R2 = 0.9147
0.6
0.5
0
8
16
24
32
40
48
56
64
Tim e (da ys)
Fig. 8. Proportion of remaining AFDW of H. opuntia in the litter bags deployed from sites I and II as a function
of time. Wo and k describes decrease in AFDW (n = 6) at site I and II during 56 days of deployment.
32
In situ calcification experiments at site I, II and III were comparable to the previous
reported by Payri, (1988). The differences between sites were possibly caused by the extent of
exposure to light between the three sites, which differed in depth and the extent of exposure
full sunlight during lowest spring tides of the day. The dark treatment did not give significant
results possibly due to two reasons; (1) dark caused the withdraw of the chloroplasts from the
exterior of the thallus to the interior (Drew and Abel, 1990), resulting into reduced
photosynthesis and calcification (2) there is a possibility that long periods of darkness resulted
into a drop in intercellular pH and a low concentration of CO32-, due to CO2 evolution during
respiration.
III: In paper III, photosynthesis and calcification processes were found to be linked
together as previously suggested by Borowitzka, (1977), and others (Pentecost, 1978; Smith
and Roth, 1979; De Beer and Larkum, 2001). The effect of AZ on C-uptake shows the
involvement extracellular carbonic anhydrase (CA) in the process of inorganic carbon uptake
as previously reported (Borowitzka and Larkum, 1976b; Hellblom, et al., 2001; Uku, 2005).
The strong effect of elevated pH in the media suggested that Halimeda calcification is
primarily driven by an increase in pH at specific regions in the thallus as suggested by
Borowitzka, (1977). The effect of DCMU as a PSII inhibitor agrees with earlier reported
studies (For example, Borowitzka and Larkum, 1979d), and it strongly blocked ETR leading
to inhibition of photosynthetic carbon uptake, and consequently decreases calcification rate
(but not a complete inhibition). However, the effect of TRIS was somewhat un-expected,
showing a strong inhibition of photosynthesis but not calcification. This suggests a structure
with both high pH alkaline and low pH acid zones, involving proton pumps. The proton
pumps from alkaline zones will favour calcification as suggested by Borowitzka (1977), while
proton pumps from alkaline zones to acid zones will result into the built-up of protons causing
low pH (which favours high CO2 concentration which diffuses into the chloroplast for
photosynthesis). The presence of TRIS will compete for proton uptake from the acid zones
causing a drop on photosynthesis, whereas the proton pumps from the alkaline zone will be
maintained, favouring high calcification rate (Fig. 9)
33
100.00
% of control
80.00
60.00
40.00
20.00
0.00
CalcLgr
GrossphotCuptakegraph
ETR
Gross O2 prod
Figure 9: Remaining activity (as % of control) in H. macroloba after addition of TRIS buffer at pH 8.2.
IV: In paper IV, red algal rhodoliths were observed to maintain high productivity and
calcification, even at combinations of both high temperature and light stress. Rates did not
decrease up to 34oC, and even at 37oC rates were high. Inhibition was observed at high
temperature above 37oC with increasing light, which suggests that algae are adapted to
survive under high temperature and light conditions. At low light level (150 μmol photons m2 -1
s ) the algae seemed to be protected from photo damage, and the reduced levels of Fv/Fm
were possibly due to photo destruction caused by oxidative stress due to D1 destruction or
other photo-destruction (Carr and Björk, unpublished). Thus, temperature tolerance seems to
be higher in rhodoliths than corals which calcify at a narrow temperature range (Marschall
and Clode, 2004). This conclude that the rhodoliths are better adapted to withstand
temperature stress, and able to live in environments with fluctuating light and Temperature
like the intertidal area of Chwaka bay where they become exposed to high light and higher
temperatures for a long period during lowest spring tides of the day.
34
CONCLUSIONS AND FUTURE PERSPECTIVES
The following conclusions were drawn from the data obtained in this study;
(1) The use of Satellite Remote Sensing technique in combination with ground-truthing
data lead into mapping Chwaka bay, and generated important estimates on changes
and percentage cover of submerged macrophytes including Halimeda species.
(2) The main Halimeda species in the area have been identified and described.
(3) There is a high standing biomass of Halimeda species in the area, high tissue C/N
ratios and Halimeda can grow up to 1 segment per tip per day.
(4) A high amount of calcium carbonate production observed indicate that the bay is
annually receiving high levels of carbonate sediments from decomposing Halimeda
materials, as reflected in their high standing biomass.
(5) Decomposition rate was slow and it could take a longer time for a given amount of
Halimeda materials to decompose into sediments.
(6) In situ calcification results were found to be comparable to the previous reports
outside the WIO region, with minor differences.
(7) The effect of metabolic inhibitors such as AZ and DCMU or varying pH on inorganic
carbon uptake by Halimeda species showed a strong inhibition on both photosynthesis
and calcification and the two, photosynthesis and calcification are linked as previously
reported. However, TRIS inhibited photosynthesis without affecting calcification,
suggesting the involvement of proton pumps, a mechanism not considered by
Borowitzka on the link between calcification and photosynthesis processes.
(8) High temperature and excess light treatments on coralline algae species (rhodoliths)
suggest that the algae are tolerant to excess light and high temperature than coral reefs,
and can maintain productivity up to 37oC.
However, more information is needed in the following areas;
(1)
Since only four Halimeda species were identified and described, there is a
possibility that some species are still unidentified from the area which needs
further taxonomical work.
(2)
This study has documented high production of carbonate sediment materials influx
in the bay. However, it is possible that most of them are exported to the main
ocean waters by the outgoing tides through the littoral fancy during ebbing. Thus,
35
further investigations are needed to document the amount of materials exported to
the main ocean with tidal currents.
(3)
Since the number of microorganisms was not quantified at the study sites during
decomposition experiments in this study, there is a possibility that in some areas,
the presence of high number of microorganisms facilitates decomposition of
Halimeda materials to decompose faster than presented here. This is an important
phenomenon for sediment generation in the area, and needs further investigation to
address the phenomenon using long term experiments.
(4)
Although the spectral and spatial resolutions of the Landsat TM/ETM+ used in this
study were appropriate, but the use of a High Resolution Multi-spectral Stereo
Imager (HRMSI) in combination with other technical advancement could yield
high quality data in the future.
Nevertheless, it’s expected that, the information contained herein, will form a baseline for
future Halimeda studies in the WIO region of East African.
ACKNOWLEDGEMENTS
This thesis could have not come into completion without the help from different people:
First and foremost, I would like to thank my supervisor, Professor Mats Björk for his tireless
guidance, advice, patience and courage even when things looked worrying! You took me
from far, since when you became a co-supervisor during my Masters Degrees studies at the
University of Dar es Salaam, all the way to Stockholm for my PhD programme. You have
been so kind beyond expectations, something I will never forget. Thank you for entrusting me
in the project, giving me both great freedom and support when I asked. Secondly, I wish to
thank Sida/SAREC for financial support during my studies from Masters Degree to PhD
level. Thank you so much the Swedish government and the Swedish people (as the main tax
payers) for your assistance to Tanzania as one among the developing countries in Africa.
Your contribution to our country is highly appreciated. I am indebted to Professor Birgitta
Bergman for her decision to allow me join the Plant Physiology section in Mats group. Your
decision is highly appreciated, without which, probably I could have not joined the Botany
Department. Thank you my co-supervisor Professor Lena Kautsky of the Department of
Botany, for your good comments and advice during writing the manuscript. Dr. Mtolera of
IMS, you are acknowledged for your favourable guidance, advice and courage especially
during field work of this project. Apart from being a co-supervisor for the Tanzania side
36
(IMS/UDSM), you also acted as a guardian, giving me good advice on how to solve my
family problems. I am grateful to Professor Phillip Bwathondi, Director General of Tanzania
Fisheries Research Institute (TAFIRI) who is my current employer, for granting me a
prolonged study leave from Masters Degree studies to PhD level. I acknowledge with thanks
to Dr. Kjell Wannäs for introducing me to Professor Bengt Lundén who made me family with
Remote Sensing and GIS techniques. I highly appreciate the efforts made by Professor Bengt
to include me in the UN course for Remote Sensing and GIS for the year 2003, and continued
to co-operate in research activities with me and my supervisor Professor Mats Björk. The
current IMS director, Dr. Dubi and the previous, Dr. Julius Francis, are acknowledged for
showing interest on me and continued to retain me at IMS from my Master’s degree studies to
PhD. I appreciate your positive recommendations you made in favour of me, in efforts to
bring me up. My close friend and fellow PhD student Alex Mamboya, thank you so much for
your courage and co-operation during the PhD programme. You always made me feel at
home during our long stay in Stockholm, sharing a lot of discussions, jokes and company. A
big hug and appreciation to my fellow PhD students from Mats group; Frida Hellblom (The
Thing), Herman Carr (Bwana Mkubwa), Jacqueline Uku and a visiting scientist Salamao
Bandeira (Ero!) who stayed with us in Mats group at the Department of Botany for one year
during 2004/2005 period as a visiting scientist. Thank you all for the good ideas, courage and
jokes during boring hours. Thank you Dimitra for your attention and act when asked for help
including going to the West Coast of Sweden to collect sea water for my experiments. The
current and previous PhD students at the Botany Department, few to mention; Pelle (Hur är
läget?), Martin, Johan Klint, Mercedes, Anders (Ecology section), Karolina, Liang, Alphonso,
Sara, Mallena, Pernilla, Jenny, Marcus Klenell, Lotta, Mathias Öster, Behnoosh and others.
Thank you all for your help of all kinds you offered to me, nice talks, company and help when
requested. Other staff members (technicians, librarians, secretaries…… etc) are
acknowledged for their positive co-operation.
To my fellow students at IMS and University of Dar es Salaam main campus, thank you
for your valuable support in different aspects, co-operation and for sharing ideas, sometime
jokes and company. I wish you good luck in your studies wherever you are registered. IMS
staffs are acknowledged for their co-operation and help of different kinds they offered to me
during my studies. I really enjoyed living with you for a long time, and I promise to
remember you all wherever I go. Specials regards goes to my “permanent technician”
Muhidin Abdallah for your patience and tireless help during my long field days in Chwaka
37
bay. You really helped me a lot, and I appreciate your contribution to my PhD degree
programme. Juma Nene and Mweleza thank you so much for your help in field work.
Lastly, to my wife Mariana Kangwe; thank you Mama for your extreme tolerance to
family problems during my absence and your strong support and courage on my Masters and
PhD studies. For many times I was away to Sweden for too long leaving you alone with the
kids in Zanzibar, but you managed to keep them healthier and joyful even in my absence.
Thank you so much Mama and god bless you.
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© Juma Walaku Kangwe
ISBN 91-7155-187-5, pp 1-47
Doctoral Thesis
Department of Botany
Stockholm University
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